He says one document is on Mexico's soon-to-be president discussing his Cabinet picks

Brazil and Mexico summoned U.S. ambassadors Monday after media reports that the United States had spied on their countries' presidents.

"Without prejudging the veracity of the information presented in the media, the Mexican government rejects and categorically condemns any espionage work against Mexican citizens in violation of international law," Mexico's foreign ministry said in a statement.

The report first appeared in Globo TV's Sunday night program "Fantastico" and is likely to heighten tensions between the United States and Latin America's two biggest economies.

One of the alleged NSA documents leaked to Greenwald dates from June 2012, a month before Mexican President Enrique Pena Nieto was elected. In it, the candidate talks about whom he would select for his Cabinet if elected.

The documents did not reference any specific communications with Brazilian President Dilma Rousseff but show the methods the NSA allegedly used to track e-mails and mobile phone communications with close advisers.

"It was very clear in the documents that they had already carried out the spying," Greenwald told "Fantastico," speaking in Portuguese. "They aren't talking about something they are planning, they are celebrating their spying successes."

Brazilian Justice Minister Eduardo Cardozo told CBN radio: "If it's confirmed, it is very serious because a country cannot passively accept the violation of its sovereignty."

"Any country that has its sovereignty violated has to react, take a position and use international law to put things in their place," he added. "And that's what Brazil will do."

A spokesman for Mexican President Enrique Peña Nieto declined to comment Monday morning.

Mexican lawmakers stressed that the Brazilian news report had not been confirmed but demanded further explanation from the U.S. government.

"This new revelation is extremely delicate because any kind of espionage is an irregular situation that is against the law. However, we have to be clear that this is speculation. This is a leak, and it must be treated like one," said Sen. Marcela Guerra.

Guerra, a member of the ruling Institutional Revolutionary Party and president of the Senate's North American foreign relations committee, said Mexico's foreign minister should meet with the U.S. ambassador to Mexico over the matter.

"There is a feeling of deep indignation," even though the report hasn't been proven, said Rep. Fernando Zarate, secretary of the Mexican house of representatives' foreign relations committee.

"If true, it seriously violates national sovereignty," said Zarate, of the Democratic Revolution Party. "How is it possible that the telephone of a president is being monitored? What could an ordinary citizen in our country expect?"

In Brazil, bilateral relations were already strained by reports thatthe South American nation was one of the countries that had been most-targeted by the NSA spying program.

Rousseff is scheduled to visit U.S. President Barrack Obama in Washington in October.

Obama visited Mexico in May, stressing the importance of strengthening educational and economic ties between the two nations.